Footing design combinations for overturning & bearing stress to Australia Standard 1

[Ey_2]

I have a relative simple question in regards to footing design
Given the allowable bearing pressure. What combinations do you use to check the stability when lateral loads and moments are involved?
For gravity I can do G+Q but I don't know what load combinations do I use for wind & earthquake?

Or do you times a factor to get ultimate bearing pressure? If so, where can I find clauses that allow us to do so?

 

[atrizzy]

For all ultimate limit states you use a factored ultimate bearing pressure.

 

[r13]

Ey2,

You should use ASD load combination, when given the "allowable bearing" pressure.

 

[Ey_2]

Thanks for the reply.

There is no ultimate bearing pressure given and that is the problem I have.

Sorry I forgot to mention that I am designing to Australian Standard where I couldn't find such ASD load combinations. I want to check overturning & stress but looks like load combination for stability load combinations are for ultimate only.

 

[r13]

Sorry to have missed the "Australian Standard" contained in the thread title. If your standard does not offer the allowable strength combination, then you might have to consult your geotechnical engineer on the limits and applicability of the "allowable bearing pressure". I would like to believe, under such environment (LRFD/limit states design only), the geotechnical engineer should have provide an ultimate bearing pressure, than the conventional allowable bearing pressure, which is derived by dividing the ultimate strength by a safety factor.

 

[r13]

Pardon my ignorance on limit state design approaches, are there different sets of load combination for strength limit state and service limit state?

 

[To Madeira]

Serviceability Limit States are the equivalent to ASD load combinations, loosely speaking. AS1170 CL4.3 includes load this which needs to be read with Table 4.1

 

[Ey_2]

Is there any factor that we normally will use to times the allowable to get ultimate? I have heard 1.3-1.35 is used but don't know where this figure comes from..

 

[To Madeira]

For soils, you'll need to check with your Geotechnical Engineer. They're notoriously conservative when it comes to safety factors, I think the FOS for allowable and ultimate soil bearing capacities is anywhere between 3 - 5 typically.

 

[Ey_2]

I am confused. If serviceability limit states is equivalent to ASD, then lets say for gravity, what combination will you choose from serviceability limit state? From what I know, there are G+0.7Q for short term and G+0.4Q for long term. But don't we normally use unfactored G+Q to compare with allowable bearing pressure? And for wind load, what combination do I use here? If I use G+0.4Q+Ws for checking the stability, won't it be 'dangerous' because I am using the Ws instead of Wu? (I understand that ultimate bearing pressure is greater than allowable one but we not only reduce the load combination factor here but also the value of wind). And for earthquake, I believe most engineers will perform ultimate analysis. I have rarely seen Es used.

 

[r13]

The minimum safety factor is at least 2, and up to 3 - 5 as addressed by Medeira. The key words to the service limit state combination is underlined below. The list given the individual un-factored load case to be considered, but there are many combinations that are possible, depending on your situations. For example, for gravity LC[sub]G[/sub] = (a)+(b), or (c); for wind, LC[sub]W[/sub] = LC[sub]G[/sub]+(d); for seismic with/without wind LC = LC[sub]W[/sub]+(e), and LC[sub]G[/sub]+(e). Since I am outside of the country of your practice, please do check my statements with engineers knowledgeable on your standard.

 

[human909]

Standard guidance as found on page 71 here:

https://wiki.csiamerica.com/downloa...steel+structures+design+manual+to+as+4100.pdf

(a) Dead load 1.35G
(b) Live load(i) 1.2G + 1.5Q
(ii) 0.9G + 1.5Q
(c) Wind load
(i) 1.2G + Wu + wcQ
(ii) 0.9G + Wu
(d) Earthquake
(i) G + wcQ + Eu

Note the 0.9G which is can be pretty critical for overturning critical condition. Obvious this is more important for tall, lightweight and slender buildings subject to wind loads. In general I find the key criteria are generally, deflection, 1.2G+1.5Q and 0.9G + Wu. Seismic in general doesn't exceed the latter two, though it can for storage or industrial type structures.

 

[rapt]

Those quoted conditions for serviceability limit states are for crack control and deflection of flexural members.

My understanding has always been that the short term loading reductions are not applicable to footing design. So it is
G + Q
G + wcG + Ws
G + wcG + E
G + Ws
G + E

Not sure why people are defining Ultimate Load Combinations for a service stress design.

r13
The idea is that you reply if you actually know the answer!

 

[Ey_2]

https://wiki.csiamerica.com/download/attachments/7...[/URL]

(a) Dead load 1.35G
(b) Live load(i) 1.2G + 1.5Q
(ii) 0.9G + 1.5Q
(c) Wind load
(i) 1.2G + Wu + wcQ
(ii) 0.9G + Wu
(d) Earthquake
(i) G + wcQ + Eu

Note the 0.9G which is can be pretty critical for overturning critical condition. Obvious this is more important for tall, lightweight and slender buildings subject to wind loads. In general I find the key criteria are generally, deflection, 1.2G+1.5Q and 0.9G + Wu. Seismic in general doesn't exceed the latter two, though it can for storage or industrial type structures.]

I believe these load combinations are for ultimate bearing stress not allowable one?

 

[Settingsun]

The 'right way' is for the geotech to give appropriate parameters, as has been said above. Limit state design isn't going away; it's effectively required by building legislation.

If that can't be obtained:
Don't use live load reductions (but you can use area reductions).

The old AS1170 gave permissible design wind speeds which were a bit higher than serviceability iirc. I may still have the document but would have to dig it out.

The old earthquake factor was 1.6, before the move to calculating a ULS action directly. But EQ design has moved forward a lot since then so treat with caution.

EQ and wind get a 33% increase in allowable pressure in a lot of references.

 

[Ey_2]

One question I have here is do we really use Ws instead of Wu here for stability check? I know that some people use ultimate combination (with Wu) and then times allowable bearing pressure a factor, which is about 1.35 to get the so-called ultimate bearing pressure to check stability. ie. 1.2G+0.3Q+Wu <-> 1.35*(allowable). So if I use G+0.3Q+Ws <-> 1.0*allowable. Looks like the later one is more aggressive because roughly Ws<Wu/1.35.
Also, for Earthquake E here, what is the service earthquake Es? Normally we just perform the ultimate analysis.

 

[Settingsun]

For stability, you have to introduce a factor of safety of 1.5 to 2 (usually).

 

[r13]

rapt,

The problem is you didn't read the op's question, and the subsequent exchanges. [Add] If you know the answer and want to be helpful, you should provide the Austraulia code provisions that indicating the footing size shall be defined by using the strength limit state forces against the "allowable bearing strength" of the soil. Otherwise, you are expressing personal bias only, not the knowledge that the OP is seeking.

In the US, this is how the "allowable bearing pressure" is derived:

"11.4.1 Ultimate Bearing Capacity of Soil
The maximum intensity of loading at the base of a foundation which causes shear failure of
soil is called ultimate bearing capacity of soil, denoted by qu.
11.4.2 Allowable Bearing capacity of Soil
The intensity of loading that the soil carries without causing shear failure and without
causing excessive settlement is called allowable bearing capacity of soil, denoted by qa. It
should be noted that qa is a service load stress. The allowable bearing capacity of soil is
obtained by dividing the ultimate bearing capacity of soil by a factor of safety on the order
of 2.50 to 3.0."

I don't know whether the Austraulia defines the terms the same way as the US.

 

[Ey_2]

I don't get this. The earthquake action in As1170.4 should be ultimate Eu. How do I get the service one Es in AS1170.0?

 

[r13]

Ey2,

I don't know if this helps, as it is an US design example. However, it may resolve your confusion on this issue. Link